Nobody should be too surprised that the Toxoplasma gondii parasite causes an alteration to human psychology. The parasite's reproductive strategy generally involves an interaction between two other species, namely cats and rodents. It works by first infecting a cat, who passes the parasite on to rats and mice through its infected feces. Infected rodents have their brains subtly rewired by the parasite, making them less scared of cats. They also start to find the smell of cat urine irresistible. These behavioral changes make it easier for cats to catch and eat the rodents, who are in turn infected by the parasite. And the cycle repeats.

While it's safe to say that we're nothing like mice, our brains do share many common fixtures. Humans clearly do not react to T. gondii in the same way that mice do — but it does seem that there may be some slight behavioral changes in humans infected with the parasite.

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If this is the case, this is no small matter: It's believed that up to one-third to half of the world's population is infected by the parasite. It is typically transmitted by handling infected cat feces, but the greater risk of infection comes through the ingestion of infected meat or the handling of unwashed vegetables and fruit.

Recent studies have shown a potential correlation to car accidents — a indication that the parasite might be causing human hosts to engage in risk taking behaviors. Research by Jaroslav Flegr, in particular, claims that the infection might increase the number of car accidents by as much as one million crashes per year, though his conclusions are highly contentious. Less controversial studies, however, have shown links between toxoplasma and schizophrenia.

And now, owing to the work of Teodor T. Postolache at the University of Maryland School of Medicine, there is stronger evidence linking T. gondii and suicide. In a study involving more than 45,000 women in Denmark, Postolache found a higher risk of attemped suicide among women who are infected. It was the largest study of its kind, and a continuation of Postolache's work which began showing a connection between the two back in 2009.

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The results of the study will appear online today, in the Archives of General Psychiatry.

Postolache cross-referenced known cases of mothers infected with toxoplasmosis with Danish health registries to determine a link, including cases of violent suicide involving guns, sharp instruments, or jumping from high places. He also made sure to know which of these women may have been diagnosed with mental illness prior to their infections.

What Postolache found was that women infected with the parasite were one and half times more likely to attempt suicide compared to those who were not infected. The risk rose with increasing levels of the T. gondii antibodies. And disturbingly, the relative risk was higher for violent suicide attempts. The researchers also concluded that prior mental illness was not a significant factor.

Unfortunately, while he was able to determine a correlation, the study did not offer any insights into potential causation. It's not immediately as to how or why there is a connection, though other studies have suggested a link with dopamine dysregulation. Postolache admits that there may be a connection between one's proclivity for acquiring T. gondii infection and an underlying psychiatric disturbances — though that sounds extremely implausible.

In addition, the study did not include men and women who didn't have children. Looking into those demographics may offer new clues into the mystery.

Is the suicide attempt a direct effect of the parasite on the function of the brain or an exaggerated immune response induced by the parasite affecting the brain? We do not know. In fact, we have not excluded reverse causality as there might be risk factors for suicidal behavior that also make people more susceptible to infection with T. gondii. If we can identify a causal relationship, we may be able to predict those at increased risk for attempting suicide and find ways to intervene and offer treatment.

Moving forward, Postolache would like to see future research focusing on molecular and behavioral factors that could better reveal a relationship between T. gondii, suicide risk factors and suicidal behavior.

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Image via Shutterstock.com/vetpathologist. Inset image courtesy of E. Prandovszky and University of Leeds.